The present invention relates to a circuit for detecting variation of resistance of a variable-resistance element, e.g., a magneto-resistive element, which varies its resistance in response to an external factor.
Among variable-resistance elements which vary the resistance in response to an external factor such as the pressure, heat and magnetic field, there is known a magneto-resistive element. The magneto-resistive element varies its resistivity in response to the external magnetic field, and is used for the reading head of magnetic card readers, magnetic tape systems and magnetic disk systems.
In the past, variation of resistance of the magneto-resistive head is detected by a circuit such as that shown, for example, in FIG. 1 (refer to Japanese Patent Application Laid-open No. 52-135706). In FIG. 1, reference number 1 denotes a 2-terminal magneto-resistive head, and 2 denotes a sense current source. The head 1 is supplied with a current by the sense current source 2, and it varies the resistance in response to the variation of the leakage magnetic flux of a recording medium. The variation of the magnetic flux produces a high frequency signal voltage, which is conducted through a derivative voltage source 3 to a pre-amplifier 7 and amplified by it. The sense current I.sub.MR flowing in the head 1 is controlled by the output of the pre-amplifier 7 and a feedback circuit 9 so that the voltage across the head 1, which is the product of the sense current I.sub.MR and the resistance R.sub.MR of the head, is equal to the voltage V.sub.OS of the derivative voltage source 3.
In this circuit arrangement, the magneto-resistive head 1 has one terminal 9 grounded for a.c. signal components, while another terminal 8 serves as a high-impedance signal line. Therefore, the terminal 8 is more susceptible than the terminal 9 to external noises existing across the head 1, producing a differential component of noise, resulting in an impaired signal-to-noise ratio of the generated signal. In one embodiment of the above patent, the derivative voltage V.sub.OS is obtained as a base-to-emitter voltage V.sub.BE of the input transistor of the pre-amplifier 7 by biasing it with a separate emitter current, and therefore it is difficult to obtain a large V.sub.OS.